Moisture barrier sealing of fiber optic coils

ABSTRACT

A method of applying a moisture barrier seal to a fiber optic coil includes mounting a fiber optic coil in a vacuum deposition chamber, so as to expose a large exterior surface area of the fiber optic coil to an interior portion of the deposition chamber. The method further includes reducing the air pressure within the chamber to a value that is less than ambient pressure outside of the chamber. The method further includes introducing a vapor form of a non-porous material, preferably parylene, into the chamber. The vapor form of the non-porous material changes into a solid state upon contact with the fiber optic coil, so as to form a conformal coat on the fiber optic coil.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0002] Not Applicable.

REFERENCE TO MICROFICHE APPENDIX

[0003] Not Applicable.

BACKGROUND OF THE INVENTION

[0004] The present invention relates to fiber optic gyroscopes(hereinafter referred to as “FOG”), and more particularly, to FOG coilsconstructed and arranged to reduce the rate of absorption of ambientmoisture.

[0005] It is well known that moisture can degrade the performance andreliability of optical fibers. Micro-cracks in the glass fiber canpropagate in the presence of ambient moisture which in turn can changethe optical properties of the fiber and potentially lead to prematurefailure. The effects of moisture depends on many factors, includingenvironmental conditions, the nature of the fiber manufacturing process,etc. Since the amount of moisture can change with environmentalconditions (e.g., temperature), the optical properties of the fiber canchange, often unpredictably, as a function of those conditions.

[0006] Fiber manufacturers typically apply an acrylate (or other similarpolymeric material) protective coating directly to the outer surface ofthe glass fiber to mitigate the effects of ambient moisture. Such acoating creates a barrier to moisture, provides some level of abrasionresistance and permits handling since bare fiber is very fragile. Whilethe coating may environmentally protect the fiber, the coating itselfmay absorb a significant amount of moisture. This phenomenon has beenobserved during bake-out procedures, i.e., when the fiber is subjectedto controlled high temperature environments, for an extended amount oftime. During bake-out procedures, coated optical fibers experience asignificant weight change (e.g., 12 percent or more), and hence adiametrical change, implying that the coating surrenders a significantamount of captured moisture while in the high temperature environment.

[0007] A coil of optical fiber is a critical component in anInterferometric Fiber Optic Gyroscope (IFOG, or more simply, FOG). A FOGis a device used to measure the rate of rotation of a vehicle or otherplatform to which the FOG is attached. The FOG typically includes a coilof optical fiber disposed about an axis of rotation. A light sourcetransmits light into each end of the optical fiber, so that two lighttransmissions propagate through the optical fiber in counter-rotatingdirections. Detection circuitry receives the light transmissions as theyemerge from the ends of the optical fiber and measures the relativephase relationship of the light. The phase relationship of the two lighttransmissions is related to the angular rotation of the FOG coil aboutthe axis of rotation, and may be used to derive an output signal that isindicative of the rate of rotation of the FOG coil.

[0008] An important parameter associated with a FOG, commonly referredto as the “scale factor,” defines and quantifies the relationshipbetween the actual rate of rotation of the FOG to the output signal ofthe FOG device (e.g., number of output pulses per arc-second ofrotation). Variations in the FOG scale factor tend to decrease theaccuracy of the FOG. The optical diameter of the fiber optic coildirectly influences the scale factor of the FOG, so any externalinfluences that could affect the optical diameter will also affect thescale factor. The optical diameter is closely related to the physicaldiameter of the coil, so any change in the physical diameter of the coilcan effect the scale factor of the FOG. Therefore, moisture absorptionby the fiber jacket directly affects the overall fiber diameter andhence the resultant scale factor of the FOG.

[0009] Epoxy materials are often applied about and between layers ofoptical fibers in the coils to provide physical stability of the windinglayers, and to maintain the coil geometry over environmental stresses.Such epoxy materials are known to be amorphous with inhomogeneities thatare commensurate with the size of water molecules, so as to permit thetransport of water molecules through capillary action. These epoxymaterials are thus hygroscopic, and if the stabilizing epoxy materialabsorbs a significant amount of moisture, the epoxy material can expandand/or deform, thus changing the coil geometry and affecting theperformance of the FOG.

SUMMARY OF THE INVENTION

[0010] In one aspect, a method of applying a moisture barrier seal to afiber optic coil comprises mounting a fiber optic coil in a vacuumdeposition chamber, so as to expose a large exterior surface area of thefiber optic coil to an interior portion of the deposition chamber. Themethod further includes reducing the air pressure within the chamber toa value that is less than ambient pressure outside of the chamber. Themethod further includes introducing a vapor form of a non-porousmaterial into the chamber. The vapor form of the non-porous materialchanges into a solid state upon contact with the fiber optic coil, so asto form a conformal coat on the fiber optic coil.

[0011] In one embodiment, the method further includes evacuating atleast some of the air within the chamber (i.e., removing air from insidethe chamber) so as to reduce the air pressure within the chamber.

[0012] In another embodiment, the method further includes heating thenon-porous material until it converts into a gaseous, vapor form.

[0013] In another embodiment, the method further includes introducingparylene vapor into the vacuum deposition chamber.

[0014] In another embodiment, the method further includes heating apredetermined quantity of parylene material until the parylene materialtransforms into a gaseous, parylene vapor.

[0015] In another embodiment, the method further includes reducing theair pressure within the chamber to a predetermined value less than theambient air pressure, wherein the predetermined value is a nominalvacuum deposition pressure.

[0016] In another aspect, a system for applying a moisture barrier sealto a fiber optic coil comprises mounting means for mounting a fiberoptic coil in a vacuum deposition chamber, so as to expose a largeexterior surface area of the fiber optic coil to an interior portion ofthe deposition chamber. The system further includes means for reducingair pressure within the chamber to a value less than ambient pressureoutside of the chamber. The system also includes means for introducing avapor form of a non-porous material into the chamber. The vapor form ofthe non-porous material changes into a solid state upon contact with thefiber optic coil, so as to form a conformal coat on the fiber opticcoil.

[0017] In one embodiment of the system, the mounting means furtherincludes mounting provisions constructed and arranged so as to expose amaximum amount of the exterior surface of the fiber optic coil to anenvironment within the deposition chamber.

[0018] In another embodiment of the system, the means for reducing airpressure further includes a vacuum pump for removing at least some airfrom within the deposition chamber.

[0019] In another embodiment, the vacuum pump reduces air pressurewithin the chamber to a predetermined value less than ambient pressure.The predetermined value is a nominal vacuum deposition pressure forapplying the non-porous material vapor to the fiber coil.

[0020] In another embodiment, the means for introducing a vapor form ofa non-porous material further includes a vapor generator for heating apredetermined amount of the non-porous material until the non-porousmaterial sublimes into a vapor form. In one embodiment, the non-porousmaterial includes parylene.

[0021] In another embodiment, the fiber optic coil remains in thechamber for a predetermined amount of time, surrounded by the vapor formof the non-porous material at the air pressure value less than ambientpressure.

[0022] In another aspect, a system for applying a moisture barrier sealto a fiber optic coil comprises a deposition chamber, a vacuum pump anda vapor generator. The deposition chamber has an access hatch fortransferring the fiber optic coil into or out of the deposition chamber,a vacuum port for transferring air into or out of the depositionchamber, and an input port for transferring deposition material into orout of the deposition chamber. The vacuum pump is attached to the vacuumport, and pumps air out of the deposition chamber, so as to reduce airpressure within the chamber to a value less than ambient pressureoutside of the chamber. The vapor generator is attached to the inputport, and introduces a vapor form of a non-porous material into thechamber. The vapor form of the non-porous material changes into a solidstate form upon contact with the fiber optic coil, so as to create aconformal coat on the fiber optic coil.

[0023] In another embodiment of the invention, the deposition chamberfurther includes mounting provisions for mounting the fiber optic coilwithin the deposition chamber. The mounting provisions may include abracket, pedestal or other similar assembly known in the art forsecuring the fiber optic coil. The mounting provisions are constructedand arranged so as to expose a large exterior surface of the fiber opticcoil to an interior portion of the deposition chamber.

[0024] In another embodiment of the invention, the vacuum pump reducesair pressure within the chamber to a predetermined value that is lessthan ambient pressure. This predetermined value of pressure in thechamber is a optimal vacuum deposition pressure, i.e., a pressure thatallows the best deposition of the non-porous material on the exteriorsurface of the optical fiber coil, without damaging the coil.

[0025] In another embodiment, the vapor generator heats a predeterminedamount of the non-porous material until the non-porous material sublimesinto a vapor form. In one embodiment, the non-porous material includesparylene.

[0026] In another embodiment of the system, the fiber optic coil remainsin the chamber for a predetermined amount of time, surrounded by thevapor form of the non-porous material at the air pressure value lessthan ambient pressure. Although material quantity and process timedefines the thickness and hence performance of the coating as a moisturebarrier, it is important that the coating is not too thick. Thickcoatings might adversely affect the performance of the FOG due todifferential thermal expansions that can induce stress on the opticalfiber.

BRIEF DESCRIPTION OF DRAWINGS

[0027] The foregoing and other objects of this invention, the variousfeatures thereof, as well as the invention itself, may be more fullyunderstood from the following description, when read together with theaccompanying drawings in which:

[0028]FIG. 1 shows a flow diagram 100 describing one embodiment a methodfor applying a moisture barrier seal to a fiber optic coil;

[0029]FIG. 2 shows a block diagram view of one preferred embodiment of asystem 200 for applying a moisture barrier seal to a fiber optic coil.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] In general, a method of applying a moisture barrier seal to afiber optic coil includes subjecting the coil to a controlled, lowpressure environment (i.e., a vacuum environment) and introducing anon-porous material, preferably in vapor form, to the low pressureenvironment along with the coil. In one embodiment, the non-porousmaterial includes parylene, although other similar materials and thincompliant films may also be used. This method allows vacuum depositionof the non-porous material on the fiber optic coil. FIG. 1 shows a flowdiagram 100 describing one embodiment a method for applying a moisturebarrier seal to a fiber optic coil. In the first step 102 of thismethod, a FOG coil assembly that has been wound via any of several coilwinding techniques known to those in the art (see, for example, U.S.patent application Ser. No. 10/003,914, entitled System and Method ofWinding a FOG Coil) is mounted in a vacuum deposition chamber, and thevacuum chamber is sealed with the coil assembly inside the chamber. Thecoil is mounted in the chamber so as to expose a maximum amount ofexterior surface area of the coil to the interior of the chamber. In oneembodiment of the invention, the FOG coil assembly is pre-treated priorto being mounted in the chamber. In general, pre-treating may includeany steps necessary to place the FOG coil in a known state. For example,if the FOG coil has been placed in storage for an extended period oftime, the coil may be holding an indeterminate amount of moisture. Oneportion of the pre-treatment may include a “bake-out” procedure, wherethe coil is placed in a controlled temperature and controlled humidityenvironment for a predetermined amount of time, so as to fix the amountof moisture the coil holds at a know value, or at least within a known,acceptable range. The FOG coil may be subjected to other such proceduresto set other various physical parameters associated with the FOG coil toknown values or ranges. In the second step 104 of the method, the vacuumpump evacuates at least some of the air from the chamber, so that thepressure within the chamber is reduced to a pressure below the ambientpressure outside of the chamber (i.e., chamber pressure is negative withrespect to ambient pressure). In other embodiments, the pressure withinthe chamber may be reduced to a pressure below ambient pressure by someother technique known in the art, for example by changing the volume orthe temperature associated with the chamber. An optimal vacuumdeposition pressure exists wherein air has been excluded from variouscavities and other generally concave regions to allow a maximum amountof the non-porous material to reach these regions, while not generatingpressure related forces that could damage the coil assembly. The actualoptimal vacuum deposition pressure depends on factors such as the natureof the coil, the amount of non-porous material to be used, etc. In thethird step 106 of the method, a predetermined amount of parylenematerial is heated until it sublimes into a gaseous monomer (i.e.,parylene vapor), and the parylene vapor is injected into the chamber. Inother embodiments of this method, a vapor form of other non-porousmaterials known in the art may be injected into the chamber instead ofthe parylene vapor. In the fourth step 108 of the method, the coilremains stationary in the chamber, at ambient temperature, for apredetermined amount of time after the introduction of the parylenevapor. The hot parylene vapor changes into a solid polymer state as itencounters the ambient temperature coil, so that the parylene polymerforms a conformal coat on the exterior surface of the coil. In a fifthstep 110 of the method, the pressure within the chamber is allowed toreturn to the ambient pressure. In the sixth step 112 of the method, thechamber is opened and the parylene-coated fiber coil is removed so thatthe system can be readied to apply a moisture barrier coating to anotherfiber coil. As a result, the surface of the coil receives a relativelythin coating of parylene to function as a moisture barrier. Therelatively thin barrier is important because thicker coatings, such asthose resulting from a more crude “potting” procedure, may adverselyaffect the performance of the FOG via differential thermal expansionsthat can induce stress on the optical fiber.

[0031]FIG. 2 shows a block diagram view of one preferred embodiment of asystem 200 for applying a moisture barrier seal to a fiber optic coil201. The system 200 includes a deposition chamber 202 with an accesshatch 203, a vacuum pump 204 attached to a vacuum port 206 on thedeposition chamber 202, a vapor generator 208 attached to an input port210 on the deposition chamber 202. The input port allows depositionmaterials to pass into and out of the chamber 202 when the chamber 202has been sealed and pressure conditions within the chamber 202 remaincontrolled. This particular embodiment of the system 200 may be used toimplement the method 100 described herein, although other similarembodiments may also be used to implement the method 100. The depositionchamber 202 includes mounting provisions 212 for securely attaching afiber optic coil within the chamber 202. The mounting provisions mayinclude a mounting bracket or pedestal or other similar means forsecurely but removably mounting the optical fiber coil within thechamber 202. The mounting provisions 212 are constructed and arranged soas to expose as much of the exterior surface of the fiber optic coil tothe environment within the deposition chamber as possible. In oneembodiment, the mounting provisions 212 in the deposition chamber 202are similar to the mounting provisions that are used to secure the fiberoptic coil within a FOG assembly, although other methods know in the artfor securing the coil within the chamber 202 may also be used.

[0032] In operation, the fiber optic coil 201 is placed into the chamber202 through the access hatch 203 and secured within the chamber 202 viathe mounting provisions 212. The access hatch 203 is then closed so asto enclose the fiber optic coil 201 in the airtight environment of thesealed chamber 202. The vacuum pump 204 then removes air from within thesealed chamber 202, so as to create at least a partial vacuumenvironment within the chamber 202. The vacuum pump 204 preferablyreduces the pressure within the chamber 202 to a pressure significantlybelow the ambient pressure outside of the chamber 202, although otherpressure levels relative to ambient may also be used. Once the pressurewithin the chamber 202 reaches a predetermined level, the vaporgenerator 208 heats solid parylene raw material until the materialsublimes into a gaseous monomer (i.e., parylene vapor). The vaporgenerator 208 introduces a predetermined amount of the hot parylenevapor into the chamber 202 via the input port 210 on the chamber 202.Other non-porous materials, similar to parylene, may also be used inother embodiments. Once the predetermined amount of parylene has beentransferred to the chamber 202, the fiber optic coil 201 remains in thechamber 202 for a predetermined amount of time (referred to herein as“soak” time), surrounded by the parylene vapor at a lower than ambientpressure. The hot parylene vapor changes into a solid polymer state asit encounters the ambient temperature coil, so that the parylene polymerforms a conformal coat on the exterior surface of the coil. After thesoak time, the vacuum pump 204 returns enough air to the chamber 202 toreturn the chamber interior to ambient pressure. The parylene-coatedfiber coil 201 is removed from chamber 202 removed so that the system200 can be readied to apply a moisture barrier coating to another fibercoil. Depending on the relative sizes of the coil and chamber, multiplecoils can be coated concurrently using the method described herein.

[0033] The invention may be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Thepresent embodiments are therefore to be considered in respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofthe equivalency of the claims are therefore intended to be embracedtherein.

What is claimed is:
 1. A method of applying a moisture barrier seal to afiber optic coil, comprising: mounting a fiber optic coil in a vacuumdeposition chamber, so as to expose a large exterior surface area of thefiber optic coil to an interior portion of the deposition chamber;reducing air pressure within the chamber to a value less than ambientpressure outside of the chamber; and, introducing a vapor form of anon-porous material into the chamber, wherein the vapor form of thenon-porous material changes into a solid state upon contact with thefiber optic coil, so as to form a conformal coat on the fiber opticcoil.
 2. A method according to claim 1, further including evacuating atleast some of the air within the chamber so as to reduce the airpressure within the chamber.
 3. A method according to claim 1, furtherincluding heating the non-porous material until it converts into agaseous, vapor form.
 4. A method according to claim 1, further includingintroducing parylene vapor into the vacuum deposition chamber.
 5. Amethod according to claim 1, further including heating a predeterminedquantity of parylene material until the parylene material transformsinto a gaseous, parylene vapor.
 6. A method according to claim 1,further including reducing air pressure within the chamber to apredetermined value less than ambient air pressure, wherein thepredetermined value is a optimal vacuum deposition pressure.
 7. A methodaccording to claim 1, further including pre-treating the fiber opticcoil so as to place the fiber optic coil in a known state.
 8. A methodaccording to claim 7, wherein pre-treating the fiber optic coil includesplacing the fiber optic coil in a controlled temperature environment fora predetermined amount of time.
 9. A method according to claim 7,wherein pre-treating the fiber optic coil includes placing the fiberoptic coil in a controlled humidity environment for a predeterminedamount of time.
 10. A system for applying a moisture barrier seal to afiber optic coil, comprising: mounting means for mounting a fiber opticcoil in a vacuum deposition chamber, so as to expose a large exteriorsurface area of the fiber optic coil to an interior portion of thedeposition chamber; means for reducing air pressure within the chamberto a value less than ambient pressure outside of the chamber; and, meansfor introducing a vapor form of a non-porous material into the chamber,wherein the vapor form of the non-porous material changes into a solidstate upon contact with the fiber optic coil, so as to form a conformalcoat on the fiber optic coil.
 11. A system according to claim 10,wherein the mounting means further includes mounting provisionsconstructed and arranged so as to expose a maximum amount of theexterior surface of the fiber optic coil to an environment within thedeposition chamber.
 12. A system according to claim 10, wherein meansfor reducing air pressure further includes a vacuum pump for removing atleast some air from within the deposition chamber.
 13. A systemaccording to claim 12, wherein the vacuum pump reduces air pressurewithin the chamber to a predetermined value less than ambient airpressure, wherein the predetermined value is a optimal vacuum depositionpressure.
 14. A system according to claim 10, wherein means forintroducing a vapor form of a non-porous material further includes avapor generator for heating a predetermined amount of the non-porousmaterial until the non-porous material sublimes into a vapor form.
 15. Asystem according to claim 10, wherein the non-porous material includesparylene.
 16. A system according to claim 10, wherein the fiber opticcoil remains in the chamber for a predetermined amount of time,surrounded by the vapor form of the non-porous material at the airpressure value less than ambient pressure.
 17. A system for applying amoisture barrier seal to a fiber optic coil, comprising: a depositionchamber having an access hatch for transferring the fiber optic coilinto or out of the deposition chamber, a vacuum port for transferringair into or out of the deposition chamber, and an input port fortransferring deposition material into or out of the deposition chamber;a vacuum pump attached to the vacuum port for pumping air out of thedeposition chamber, so as to reduce air pressure within the chamber to avalue less than ambient pressure outside of the chamber; and, a vaporgenerator attached to the input port for introducing a vapor form of anon-porous material into the chamber, wherein the vapor form of thenon-porous material changes into a solid state form upon contact withthe fiber optic coil, so as to create a conformal coat on the fiberoptic coil.
 18. A system according to claim 17, wherein the depositionchamber further includes mounting provisions for mounting the fiberoptic coil within the deposition chamber, so as to expose a largeexterior surface of the fiber optic coil to an interior portion of thedeposition chamber.
 19. A system according to claim 18, wherein themounting means further includes mounting provisions constructed andarranged so as to expose a maximum amount of the exterior surface of thefiber optic coil to an environment within the deposition chamber.
 20. Asystem according to claim 17, wherein the vacuum pump reduces airpressure within the chamber to a predetermined value less than ambientpressure, the predetermined value being a optimal vacuum depositionpressure.
 21. A system according to claim 17, wherein the vaporgenerator heats a predetermined amount of the non-porous material untilthe non-porous material sublimes into a vapor form.
 22. A systemaccording to claim 17, wherein the non-porous material includesparylene.
 23. A system according to claim 17, wherein the fiber opticcoil remains in the chamber for a predetermined amount of time,surrounded by the vapor form of the non-porous material at the airpressure value less than ambient pressure.
 24. A system for applying amoisture barrier seal to a fiber optic coil, comprising: a depositionchamber having an access hatch for transferring the fiber optic coilinto or out of the deposition chamber, a vacuum port for transferringair into or out of the deposition chamber, an input port fortransferring deposition material into or out of the deposition chamber,and mounting provisions for mounting the fiber optic coil within thedeposition chamber, the mounting provisions constructed and arranged soas to expose a maximum amount of the exterior surface of the fiber opticcoil to an environment within the deposition chamber; a vacuum pumpattached to the vacuum port for pumping air out of the depositionchamber, so as to reduce air pressure within the to a predeterminedvalue less than ambient pressure, the predetermined value being aoptimal vacuum deposition pressure; and, a vapor generator attached tothe input port for introducing parylene vapor into the chamber, whereinthe parylene vapor changes into a solid state form upon contact with thefiber optic coil, so as to create a conformal coat on the fiber opticcoil; wherein the fiber optic coil remains in the chamber for apredetermined amount of time, surrounded by the vapor form of paryleneat the air pressure value less than ambient pressure.